Fiberglass is currently produced from glass-fiber bushings that typically are controlled to maintain the temperature of the glass contained in the forming restrictions (tips) within a desired range. Maintenance of a desired glass temperature insures uniform viscosity of the molten glass in a forming restrictions (tips) and thus uniform filament formation as fibers are drawn from the molten glass as it exits through the forming restrictions, e.g., bushing orifices, all other conditions being equal. The bushings themselves are precious metal containers typically having four sides and a bottom and are generally open at the top. The bushings are provided with a plurality of forming restrictions, e.g., orifices, in the bottom which normally have tips depending therefrom through which the molten glass contained in the bushing flows to form the glass fibers. Bushings are heated by applying current to leads located at each end of the bushing, the leads being connected to the secondary winding of a suitably sized power transformer. The current is increased or decreased by connecting a power pack to the power transformer and altering the output of the power pack by feeding signals to it from a controller that is fed signals from thermocouples attached to the bushing that monitor its operating temperatures continuously.
The throughput of a glass-fiber bushing, under stable operating conditions, is determined by the average temperature of the glass in the forming restrictions. A common technique for determining the temperature of the glass in the forming restrictions is by the use a thermocouple placed on the bushing's front side wall, close to the tip plate (bottom). While the present technique utilized to measure bushing temperatures appears adequate in that the temperature does represent the measurements taken from the bushing, they are not necessarily adequate because they do not necessarily represent the real temperature of the forming restrictions or of the tip-plate. This is so because conventional measurements made do not usually represent the bushing tip-plate temperature. It will be appreciated that substantial temperature gradients exist within the bushing, so this measurement only approximates the temperature in the tips. Also, because the thermocouple is located away from the tips, there is a lag between changes observed in the side wall and throughput changes occurring in the forming restrictions.
Accordingly, it is an object of the present invention to improve the temperature measurement of a glass-fiber bushing at the tip-plate. In measuring the temperature of the bushing tip-plate, it is an important consideration that the thermocouple junction be attached to the top of the tip-plate of the bushing. The present invention provides improved temperature measurement by placing the point of measurement on the tip-plate. Consequently, the present invention provides improved speed of response, less cold-start or hot-start yardage and reduced variability in the yardage of the fiber glass produced.